tcpkeepalive: Support CURLOPT_TCP_KEEPIDLE on OSX

MacOS X doesn't have TCP_KEEPIDLE/TCP_KEEPINTVL but only a single
TCP_KEEPALIVE (see
http://developer.apple.com/library/mac/#DOCUMENTATION/Darwin/Reference/ManPages/man4/tcp.4.html).
Here is a patch for CURLOPT_TCP_KEEPIDLE on OSX platforms.